1. Field of the Invention
The present invention relates to an electron emitting device, an electron source, and an image forming apparatus.
2. Related Background Art
Conventionally, two types of electron sources (cathodes), that is, a thermionic and a cold cathode have been known as the electron emitting device. As the cold cathode, there are a field emission type (hereinafter referred to as an FE type) electron emitting device, a metal/insulating-layer/metal type (hereinafter referred to as an MIM type) electron emitting device, a surface conduction type electron emitting device, or the like.
As examples of the FE type, those disclosed in W. P. Dyke and W. W. Dolan, xe2x80x9cField Emissionxe2x80x9d, Advance in Electron Physics, 8, 89 (1956), C. A. Spindt, xe2x80x9cPhysical Properties of thin-film field emission cathodes with molybdenum conesxe2x80x9d, J. Appl. Phys., 47, 5248 (1976), and the like have been known.
As examples of the MIM type, the one as disclosed in C. A. Mead, xe2x80x9cOperation of Tunnel-Emission Devicesxe2x80x9d, J. Appl. Phys., 32, 646 (1961), and the like have been known.
Also, as recent examples, Toshiaki Kusunoki, xe2x80x9cFluctuation-free electron emission from non-formed metal-insulator-metal (MIM) cathodes fabricated by low current anodic oxidationxe2x80x9d, Jpn. J. Appl. Phys. Vol. 32 (1993) pp. L1695, Mutsumi Suzuki, et.al, xe2x80x9cAn MIM-cathode array for cathode luminescent displaysxe2x80x9d, IDW""96, (1996) pp. 529, and the like have been studied.
As examples of the surface conduction type, there are the ones as described in Elinson""s report (M. I. Elinson, Radio Eng. Electron Phys., 10 (1965)), and the like. This surface conduction type electron emitting device is realized by utilizing the phenomenon that electrons are emitted out of a small area thin film formed on a substrate when a current is made to flow in parallel with the film surface. As the surface conduction type electron emitting device, the device using an SnO2 thin film described in the above Elinson""s report, a device using an Au thin film (G. Dittmer. Thin Solid Films, 9, 317 (1972)), a device using an In2O3/SnO2 thin film (M. Hartwell and C. G. Fonstad: IEEE Trans. ED Conf., 519 (1983)), and the like, have been reported.
When the electron emitting device is applied to the image forming apparatus (in particular, the display), it is necessary to obtain an emitting current for causing a phosphor to emit light with a sufficient intensity. Also, for high minuteness of the display, it is desired that a diameter of an electron beam to be irradiated into the phosphor is small. Also, it is important to make the manufacturing easy.
As an example of a conventional FE type, a so-called xe2x80x9cspindt typexe2x80x9d electron emitting device is shown in FIG. 29. In FIG. 29, reference numeral 1 denotes a substrate, 4 denotes a cathode electrode layer (lower potential electrode), 3 denotes an insulating layer, 2 denotes a gate electrode layer (higher potential electrode), 5 denotes a microchip, and 6 denotes an equipotential surface. When a bias is applied between the microchip 5 having a curvature xe2x80x9crxe2x80x9d and the gate electrode layer 2, electrons are emitted from the end of the microchip 5 toward an anode. An amount of emitting electron is determined by the distance xe2x80x9cdxe2x80x9d between the gate electrode layer 2 and the end of the microchip 5, a voltage Vg between the gate electrode and the microchip, a work function of an emitting region material (microchip), and the like. Namely, that the device is manufactured by controlling the distance xe2x80x9cdxe2x80x9d between the gate electrode layer 2 and the microchip 5 is a factor for determining the performance of the device.
A general manufacturing process of the Spindt type electron emitting device is shown in FIGS. 30A to 30D. The manufacturing process will be described through those drawings. First, the cathode electrode layer 4 made of Nb or the like, the insulating layer 3 made of SiO2 or the like, and the gate electrode layer 2 made of Nb or the like are laminated in this order on the substrate 1 made of glass or the like. Then, a circular minute hole which penetrates the gate electrode layer 2 and the insulating layer 3 is formed by a reactive ion etching method (FIG. 30A).
After that, a sacrificial layer 7 made of aluminum or the like is formed on the gate electrode layer 2 by oblique evaporation or the like (FIG. 30B).
A microchip material 8 such as molybdenum is deposited in the structure formed thus by a vacuum evaporation method. Here, the minute hole is filled with the deposition on the sacrificial layer by the progress of deposition. Thus, the microchip 5 is conically formed in the minute hole (FIG. 30C).
Finally, the sacrificial layer 7 is dissolved to lift off the microchip material 8. Thus, the device is completed (FIG. 30D).
However, in such a manufacturing method, it is difficult to control the distance xe2x80x9cdxe2x80x9d with high repeatability. Thus, there is the case where a variation in an amount of emitting current between devices is produced by a variation in the distance xe2x80x9cdxe2x80x9d. Also, if the device is driven with the state that a short circuit occurs between the microchip 5 and the gate electrode layer 2 through a piece of metal or the like which is produced by the lift off, there is the case where heat is generated in the shirt circuit region and thus a discharge breakdown occurs in the shirt circuit region and its surrounding. In this case, an effective electron emitting region is decreased. Thus, in an image forming apparatus (in particular, a display) using a plurality of devices having the above variation in an amount of emitting electrons, an unevenness of brightness occurs. Thus, the apparatus becomes a low performance as the display.
Further, in the Spindt type device, electrons are emitted from an extremely narrow region. Thus, when an emitting current density is increased in order to cause the phosphor to emit light, there is the case where a thermal breakdown of the electron emitting region (microchip) is induced, and thus the life of the device is limited. Also, these is the case where the end of the microchip is intensively sputtered with ions present in a vacuum, and thus the life of the device is shortened.
Note that electrons emitted to the vacuum are carried along the direction orthogonal to an equipotential surface. However, in the structure as shown in FIG. 29, the equipotential surface 6 is formed in the hole along the outer shape of the microchip 5. Thus, the electrons emitted from the end of the microchip 5 tend to spread. Since a portion of the emitted electrons is absorbed into the gate electrode layer 2, an amount of electrons which reach the anode is decreased. When the distance xe2x80x9cdxe2x80x9d is shortened, an amount of electrons absorbed in the gate electrode layer 2 tends to increase.
In order to overcome such faults, various examples have been proposed.
As an example for preventing the diffusion of an electron beam, there is one that a focusing electrode 9 is located over the electron emitting region. FIG. 31 is a structure view of an FE type device with the focusing electrode. In this example, the emitted electron beam is focused with the potential of the focusing electrode 9. However, this example requires a further complicated process than the above manufacturing process, and thus increase in a manufacturing cost occurs.
As an example for reducing the diameter of an electron beam without locating the focusing electrode, there is the one described in Japanese Patent Application Laid-Open No. 8-264109. This structure is shown in FIG. 32. In this example, in order to emit electrons from a thin film 10 located in a hole, since a flat equipotential surface 6a is formed on an electron emitting surface, the diffusion of the electron beam becomes small. However, in this example, since the electron emitting region is present in the hole and the gate electrode layer 2 is located over the electron emitting surface as conventionally, a potential distribution 6b correlated with the depth of the hole and a gate electrode interlayer distance is formed in the vicinity of the hole. Therefore, although not to the extent of the spindt type, the emitted electrons tend to spread and thus the problem in that a portion of the emitted electrons is absorbed into the gate electrode layer 2 is not solved.
As an example for improving an electron emitting efficiency, there are those described in Japanese Patent Application Laid-Open No. 10-289650, U.S. Pat. No. 6,135,839 and the like. The structure is shown in FIG. 33. A positive potential (voltage) is applied to a gate electrode layer 2 and a second gate electrode layer 11 with reference to a cathode electrode layer 4 (where 0 less than |Vg1|xe2x89xa6|Vg2|) and thus an amount of electron emitted from the cathode electrode layer 4 is increased. However, even in this example, the emitted electrons tend to spread.
Similarly, as an example for improving the electron emitting efficiency, there is a report that a needle shaped electrode is located in a minute hole formed by Al anodic oxidation, and thus a density in a cathode electrode and an amount of emitting electron per unit area are increased (Japanese Patent Application Laid-Open No. 5-211029).
However, even in this example, the emitted electrons tend to spread. Thus, a complicated manufacturing method is required such that the cathode electrode is located in the minute hole.
On the other hand, as shown in FIG. 34, the MIM type has the structure in that an insulating layer 3 is located between a lower electrode (cathode electrode layer) 4 and an upper electrode (gate electrode layer) 2, and a voltage is applied between both electrodes 4 and 2 to lead the electrons. In this structure, since the direction of an internal electric field coincides with that of emitting electrons, and a potential distribution on an emitting surface is not disturbed, a small diameter of an electron beam can be realized. However, since scattering of electrons is produced in the insulating layer 3 and the upper electrode 2, the efficiency is generally low.
An example of the conventional surface conduction type electron emitting device is shown in FIG. 35 (although the electron emitting devices until here are shown with the cross sectional views, this example is shown with a plane view). In FIG. 35, reference numeral 1 denotes a substrate, 4 denotes a device cathode (cathode electrode layer), 2 denotes a device anode (gate electrode layer), 23 denotes electroconductive film, and 24 denotes an electron emitting region. Even in the surface conduction type electron emitting device, generally, a relationship between the electron emitting efficiency and the diameter of the electron beam is a trade-off. As respective solving methods, there are the proposal with respect to high efficiency (Japanese Patent Application Laid-Open No. 9-82214), the proposal with respect to the convergence of electron beam (Japanese Patent Application Laid-Open No. 2-112125), and the like.
An example that the electron emitting device is applied as an image forming apparatus is shown in FIG. 36. In this example, lines of gate electrodes layers 2 and lines of cathode electrode layers 4 are arranged in a matrix, electron emitting devices 14 are arranged in cross sectional portions of both lines. In response to an information signal, electrons are emitted from the electron emitting device 14 located in the selected cross sectional portion, and accelerated by the voltage of an anode 12. Thus, the electrons are incident to the phosphors 13. This is a so-called triode type device.
Also, as shown in FIG. 37, there is a tetrode type structure in that modulation electrodes 15 (referred to as grids) is added between electron emitting deices 14 and an anode 12, and a voltage corresponding to an information signal is applied to these electrodes to control an electron flow from the electron emitting device 14.
As the tetrode type, in order to improve the alignment and the arrangement of the modulation electrode 15 with the electron emitting devices 14, there is a proposal that, as shown in FIGS. 38 and 39 (FIG. 39 is a cross sectional view along a line 39xe2x80x9439 in FIG. 38), modulation electrodes 15 are arranged in a rear side against electron emitting devices 14 through an insulating layer 3 (for example, Japanese Patent Application Laid-Open No. 3-20941).
In the case where the above electron emitting device is applied to an image forming apparatus such as a display, it is required that,
(1) a diameter of the electron beam is small,
(2) an electron emitting area is large,
(3) electrons can be emitted with a low voltage and high efficiency, and
(4) a manufacturing process is easy.
However, it is difficult to simultaneously satisfy these requirements in a conventional electron emitting device.
The present invention is made to solve the above problems, and therefore an object of the present invention is to provide an electric field emitting type electron emitting device, an electron source, an image forming apparatus, and the electron emitting apparatus, in which the diameter of the electron beam is small, the electron emitting area is large, the electron emitting can be made with a low voltage and high efficiency, and the manufacturing process is easy.
To achieve the above object, an electron emitting device of the present invention is characterized by comprising: a first electrode located on a substrate; an insulating layer located on the first electrode; and a second electrode located on the insulating layer, whereby the second electrode has a first surface and a second surface, which are substantially vertical to a direction that the first electrode and the insulating layer are laminated, the first surface of the second electrode is in contact with the insulating layer, and a higher potential than that applied to the second electrode is applied to the first electrode to emit an electron from the second surface.
Also, to achieve the above object, an electron emitting device of the present invention is characterized by comprising: a first electrode located on a substrate; an insulating layer located on the first electrode; and a second electrode located on the insulating layer, whereby the second electrode has a first surface in contact with the insulating layer and a second surface opposite to the first surface, and a higher potential than that applied to the second electrode is applied to the first electrode to emit an electron from the second surface.
Therefore, when the anode is located opposite to the electron emitting device of the present invention and thus the electron emitting apparatus or the image forming apparatus is manufactured, the equipotential surface between the electron emitting device and the anode is substantially parallel to the anode surface and a uniform potential distribution is formed. Thus, electrons emitted to a vacuum are moved toward the anode, and the diffusion of the electron beam can be suppressed. As a result, the diameter of the electron beam can be made small.
Also, the electron emitting area corresponds to the surface of the cathode electrode to which a low potential is applied in the anode side. Thus, since the electron emission area is wide, the durability to the bombardment of ions present in a vacuum is high.
Further, obstacles which prevent the trajectory of electrons toward the anode and a potential that produces obstacles, are not present. Thus, since almost all the emitting electrons become an emission current, the electron emission can be made with a low voltage and high efficiency.
Then, a very simple structure in that the gate electrode (to which a high potential is applied), the insulating layer, and the cathode electrode (to which a low potential is applied) are laminated on the substrate in this order, is obtained, and the manufacturing process is easy.
Thus, according to a field emission type electron emitting device with a characteristic of the present invention, since, the diameter of the electron beam is small, the electron emission area is large, the electron emission can be made with a low voltage and high efficiency, and the manufacturing process is easy, such a device can be applied to the image forming apparatus such as a display.
Therefore, the electron emitting apparatus, the electron source, and the image forming apparatus, to which the electron emitting device of the present invention is applied, can be realized with a high performance.